A. Field of Invention
This invention pertains to transcutaneous tissue stimulating devices such as cochlear prostheses and functional electrical stimulators. Such devices are frequently of a two-part design, there being a part implanted within the body of the patient, for the purpose of providing therapy, and an external component in communication with the implanted part by means of a transcutaneous wireless link.
B. Description of the Prior Art
Implantable prosthesis devices are generally required to be small to reduce the trauma and other complications arising from the implantation and maintenance of foreign matter inside a patient's body. For example, cochlear implant systems typically include an external component and an internal component. The external component includes a microphone for sensing ambient sounds and a signal processor for generating stimulation signals corresponding to these sounds. The stimulation signals are transmitted to the internal component which applies the same to the aural nerve of the patient through an array of electrodes. Because the internal component may not have a permanent power supply, power for the component is also derived from the external component.
The means for providing both power and communication between the internal and external components is typically a pair of coils, one in the external component and one in the internal component. The coils are planar and are positioned parallel to each other so that energy is coupled through the skin and flesh of the patient from the external coil to the internal coil for the purpose of powering and/or controlling the internal component. Until now, the preferred arrangement for these coils was such that the voltage induced into the internal coil was very sensitive to variations in inter-coil distance.
Since the coils are separated by a partition comprising skin and tissue, the inter-coil distance is not constant from patient-to-patient but varies according to the anatomical characteristics of each implantee. A ramification of this variability is that in order to ensure that an adequate voltage level is induced in the internal coil, for purposes of operating the internal component of the cochlear implant, the voltage applied to the external coil must be sufficiently high to take into account patients having considerably thicker or thinner partition widths than typically encountered. As a consequence the voltage applied by the transmit coil is set to a relatively high value in order that the implanted portion will be supplied with sufficient power no matter the width of the implanteeOs partition. Accordingly, an undesirable power wastage occurs in the majority of patients. A cochlear implant system of this kind is described in U.S. Pat. No. 4,532,930.
In a related known arrangement, tuned or tank circuits are used in the transmitter and receiver to transmit signals and power. In this arrangement, the critical parameter for power transmission is the coupling coefficient between the tuned circuit coils. This coupling coefficient is optimal only at a critical spacing between coils. One such system is disclosed in U.S. Pat. No. 4,654,880 to Sontag. Again, for optimal coupling, the two coils must be spaced at a critical distance from each other and must be co-axial. If the coils are not at the optimal distance, the signals are exchanged between the components at a low efficiency.